3GPP LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme. In a radio communication system to which 3GPP LTE is applied, a base station transmits a synchronization signal (Synchronization Channel: SCH) and broadcast signal (Broadcast Channel: BCH) using predetermined communication resources. A terminal secures synchronization with the base station by catching an SCH first. After that, the terminal acquires parameters specific to the base station (e.g., frequency bandwidth) by reading BCH information (see Non-Patent Literatures 1, 2 and 3).
Furthermore, after completing the acquisition of parameters specific to the base station, the terminal makes a connection request to the base station to thereby establish communication with the base station. The base station transmits control information to the terminal with which communication is established via a PDCCH (Physical Downlink Control CHannel) as required.
The terminal then makes a “blind decision” on each of a plurality of pieces of control information included in the received PDCCH signal. That is, the control information includes a CRC (Cyclic Redundancy Check) portion and this CRC portion is masked with a terminal ID of the transmission target terminal in the base station. Therefore, the terminal cannot decide whether or not the control information is directed to the terminal until the CRC portion of the received control information is demasked with the terminal ID of the terminal. When the demasking result shows that the CRC calculation is OK in the blind decision, the control information is decided to be directed to the terminal.
Furthermore, in 3GPP LTE, ARQ (Automatic Repeat Request) is applied to downlink data from a base station to a terminal. That is, the terminal feeds back a response signal indicating the error detection result of the downlink data to the base station. The terminal performs a CRC on the downlink data and feeds back ACK (Acknowledgment) when CRC=OK (no error) and NACK (Negative Acknowledgment) when CRC=NG (error present) as a response signal to the base station. An uplink control channel such as PUCCH (Physical Uplink Control Channel) is used for feedback of this response signal (that is, ACK/NACK signal).
Here, the control information transmitted from the base station includes resource allocation information including resource information or the like allocated by the base station to the terminal. The aforementioned PDCCH is used for transmission of this control information. This PDCCH is made up of one or a plurality of L1/L2 CCHs (L1/L2 Control Channels). Each L1/L2 CCH is made up of one or a plurality of CCEs (Control Channel Elements). That is, a CCE is a base unit when control information is mapped to a PDCCH. Furthermore, when one L1/L2 CCH is made up of a plurality of CCEs, a plurality of continuous CCEs are allocated to the L1/L2 CCH. The base station allocates an L1/L2 CCH to the resource allocation target terminal according to the number of CCEs necessary to report control information for the resource allocation target terminal. The base station then transmits control information mapped to physical resources corresponding to the CCEs of the L1/L2 CCH.
Here, each CCE has a one-to-one correspondence with a component resource of the PUCCH. Therefore, the terminal that has received the L1/L2 CCH identifies component resources of the PUCCH corresponding to CCEs making up the L1/L2 CCH and transmits a response signal to the base station using the resources. However, when a plurality of CCEs where there are continuous L1/L2 CCHs are occupied, the terminal transmits a response signal to the base station using one of the plurality of PUCCH component resources (e.g., PUCCH component resources corresponding to a CCE having the smallest index) corresponding to the plurality of respective CCEs. This allows downlink communication resources to be used efficiently.
As shown in FIG. 1, a plurality of response signals transmitted from a plurality of terminals are spread by a ZAC (Zero Auto-correlation) sequence having a Zero Auto-correlation characteristic, Walsh sequence and DFT (Discrete Fourier Transform) sequence on the time axis and code-multiplexed within the PUCCH. In FIG. 1, (W0, W1, W2, W3) represents a Walsh sequence having a sequence length of 4 and (F0, F1, F2) represents a DFT sequence having a sequence length of 3. As shown in FIG. 1, in the terminal, a response signal such as ACK or NACK is primary-spread by a ZAC sequence (sequence length 12) into a frequency component corresponding to 1 SC-FDMA symbol on the frequency axis first. Next, the primary-spread response signal and the ZAC sequence as a reference signal are secondary-spread in association with a Walsh sequence (sequence length 4: W0 to W3) and DFT sequence (sequence length 3: F0 to F3) respectively. Furthermore, the secondary-spread signal is further transformed into a signal having a sequence length of 12 on the time axis through IFFT (Inverse Fast Fourier Transform). A CP is added to each signal after the IFFT and a one-slot signal made up of seven SC-FDMA symbols is thereby formed.
Response signals transmitted from different terminals are spread using a ZAC sequence corresponding to different cyclic shift indices or orthogonal code sequences corresponding to different sequence numbers (Orthogonal cover Index: OC index). The orthogonal code sequence is a combination of a Walsh sequence and a DFT sequence. Furthermore, the orthogonal code sequence may be referred to as a “block-wise spreading code.” Therefore, the base station can demultiplex a plurality of code-multiplexed response signals using conventional despreading and correlation processing (see Non-Patent Literature 4).
However, since each terminal makes a blind decision on a downlink allocation control signal directed to the terminal in each subframe, the terminal side does not necessarily succeed in receiving the downlink allocation control signal. When the terminal fails to receive the downlink allocation control signal directed to the terminal in a certain downlink unit band, the terminal cannot even know whether or not there is downlink data directed to the terminal in the downlink unit band. Therefore, when failing to receive the downlink allocation control signal in a certain downlink unit band, the terminal cannot even generate a response signal for the downlink data in the downlink unit band. This error case is defined as a DTX of response signal (DTX (Discontinuous transmission) of ACK/NACK signals) in the sense that transmission of the response signal is not performed on the terminal side.
Furthermore, standardization of 3GPP LTE-advanced which realizes faster communication than 3GPP LTE has started. A 3GPP LTE-advanced system (hereinafter, may also be referred to as “LTE-A system”) follows the 3GPP LTE system (hereinafter also referred to as “LTE system”). In order to realize a downlink transmission rate of a maximum of 1 Gbps or above, 3GPP LTE-advanced is expected to introduce base stations and terminals capable of communicating at a wideband frequency of 40 MHz or above.
In an LTE-A system, to realize communication at an ultra-high transmission rate several times as fast as the transmission rate in an LTE system and backward compatibility with the LTE system simultaneously, a band for the LTE-A system is divided into “unit bands” of 20 MHz or less, which is a support bandwidth for the LTE system. That is, the “unit band” is a band having a width of maximum 20 MHz and defined as a base unit of a communication band. Furthermore, a “unit band” in a downlink (hereinafter referred to as “downlink unit band”) may be defined as a band divided by downlink frequency band information in a BCH broadcast from the base station or by a spreading width when the downlink control channel (PDCCH) is spread and arranged in the frequency domain. On the other hand, a “unit band” in an uplink (hereinafter referred to as “uplink unit band”) may be defined as a band divided by uplink frequency band information in a BCH broadcast from the base station or as a base unit of a communication band of 20 MHz or less including a PUSCH (Physical Uplink Shared CHannel) region near the center and PUCCHs for LTE at both ends. Furthermore, in 3GPP LTE-Advanced, the “unit band” may also be expressed as “component carrier(s)” in English.
The LTE-A system supports communication using a band that bundles several unit bands, so-called “carrier aggregation.” Since throughput requirements for an uplink are generally different from throughput requirements for a downlink, in the LTE-A system, studies are being carried out on carrier aggregation using different numbers of unit bands set for an arbitrary LTE-A system compatible terminal (hereinafter referred to as “LTE-A terminal”) between the uplink and downlink, so-called “asymmetric carrier aggregation.” Cases are also supported where the number of unit bands is asymmetric between the uplink and downlink and the frequency bandwidth differs from one unit band to another.
FIG. 2 is a diagram illustrating asymmetric carrier aggregation and its control sequence applied to individual terminals. FIG. 2 shows an example where the bandwidth and the number of unit bands are symmetric between the uplink and downlink of a base station.
In FIG. 2, a setting (configuration) is made for terminal 1 such that carrier aggregation is performed using two downlink unit bands and one uplink unit band on the left side, whereas a setting is made for terminal 2 such that although the two same downlink unit bands as those in terminal 1 are used, the uplink unit band on the right side is used for uplink communication.
Focusing attention on terminal 1, signals are transmitted/received between an LTE-A base station and LTE-A terminal making up an LTE-A system according to the sequence diagram shown in FIG. 2A. As shown in FIG. 2A, (1) terminal 1 establishes synchronization with the downlink unit band on the left side at a start of communication with the base station and reads information of the uplink unit band which forms a pair with the downlink unit band on the left side from a broadcast signal called “SIB2 (System Information Block Type 2).” (2) Using this uplink unit band, terminal 1 starts communication with the base station by transmitting, for example, a connection request to the base station. (3) Upon deciding that a plurality of downlink unit bands need to be allocated to the terminal, the base station instructs the terminal to add a downlink unit band. In this case, however, the number of uplink unit bands does not increase and terminal 1 which is an individual terminal starts asymmetric carrier aggregation.
Furthermore, in LTE-A to which the aforementioned carrier aggregation is applied, the terminal may receive a plurality of pieces of downlink data in a plurality of downlink unit bands at a time. In LTE-A, studies are being carried out on channel selection (also referred to as “multiplexing”) as one of transmission methods for a plurality of response signals for the plurality of pieces of downlink data. In channel selection, not only symbols used for a response signal but also resources to which the response signal is mapped are changed according to a pattern of error detection results regarding the plurality of pieces of downlink data. That is, channel selection is a technique that changes not only phase points (that is, constellation points) of a response signal but also resources used to transmit the response signal based on whether each of response signals for a plurality of pieces of downlink data received in a plurality of downlink unit bands as shown in FIG. 3 is ACK or NACK (see Non-Patent Literatures 5 and 6).
Here, ARQ control by channel selection when the above-described asymmetric carrier aggregation is applied to a terminal will be described using FIG. 3.
When, for example, a unit band group made up of downlink unit bands 1 and 2, and uplink unit band 1 (which may be expressed as “component carrier set” in English) is set for terminal 1 as shown in FIG. 3, downlink resource allocation information is transmitted from the base station to terminal 1 via respective PDCCHs of downlink unit bands 1 and 2 and then downlink data is transmitted using resources corresponding to the downlink resource allocation information.
When the terminal succeeds in receiving downlink data in unit band 1 and fails to receive downlink data in unit band 2 (that is, when the response signal of unit band 1 is ACK and the response signal of unit band 2 is NACK), the response signal is mapped to PUCCH resources included in PUCCH region 1 and a first constellation point (e.g., constellation point (1,0)) is used as a constellation point of the response signal. On the other hand, when the terminal succeeds in receiving downlink data in unit band 1 and also succeeds in receiving downlink data in unit band 2, the response signal is mapped to PUCCH resources included in PUCCH region 2 and the first constellation point is used. That is, when there are two downlink unit bands, since there are four error detection result patterns, the four patterns can be represented by combinations of two resources and two types of constellation point.